Electrical restitution is the relationship between changes in action potential duration with varying diastolic intervals occurring between a first cardiac systole and an extra systole. Restitution reflects the recovery properties of the cardiac tissue with respect to the time of initiation of the extra systole. An electrical restitution curve can be constructed by measuring the action potential duration over a range of diastolic intervals. The curve is initially very steep where short diastolic intervals result in a greater shortening of the action potential durations. After the initially steep portion, a plateau is reached as the action potential duration (APD) reaches a maximum at longer diastolic intervals. The slope of the electrical restitution curve over the entire range of diastolic intervals or the slope of the steepest portion of the curve can be used as a measure of the responsiveness of APD changes to a change in diastolic interval. Increased dispersion of action potential duration and refractoriness is associated with an increased risk of arrhythmias. The shortened action potential duration resulting from the shortened diastolic interval alters the refractoriness of the myocardium which is believed to set up pathways for reentrant depolarizations in a way that greatly enhances susceptibility to fibrillation.